High tensile strength electrodeposited copper foil

ABSTRACT

A high tensile strength copper foil having a matte side roughness Rz of 2.5 μm or less and a tensile strength of 40,000 Kgf/mm 2  after heating one hour at 180° C. is produced by a process in which copper is electrodeposited from a solution containing predetermined small amounts of a polyether glycol and tin and iron ions. The chloride ion content is maintained below 0.1 wt. ppm. The copper foil is characterized by having copper crystals with atomic cells having a predominently (111) orientation.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.08/855,316, filed May 13, 1997, now U.S. Pat. No. 5,958,209, whichclaims priority from Japanese Application No. Hei 8-141198, filed May13, 1996.

BACKGROUND OF THE INVENTION

This invention relates to electrodeposited copper foils such as are usedto make “printed” circuits. In particular, the invention relates to acopper foil having a high tensile strength even after heating andcharacterized by having a low profile matte side and a unique crystalorientation, which provides more precisely etched circuit lines.

Electrodeposited copper foils are made by electrolysis of a coppersolution. Typically, copper metal is deposited on a rotating metal drumwhich serves as the cathode. After the desired thickness has beenachieved the copper foil is removed from the drum and givenpost-treatments to protect it and to improve adhesion to a substrate.Then, the foil is wound into rolls for shipment to the user, where it islaminated to an insulating substrate, such as a glass fiber reinforcedepoxy resin, and then photoimaged and etched to produce the desiredcircuit pattern. The etching process is critical to the actualperformance of the circuit. Ideally, only copper not intended to be apart of the circuit pattern should be removed by the chemicals used toetch the copper. In fact, the copper is removed irregularly and insteadof circuit lines having sharply defined sides, they are typicallytrapezoidal in shape. The top of the circuit lines are usually narrowerthan intended and the bottom wider. Furthermore, small amounts of coppermay be left embedded in the substrate which are in electrical contactwith the circuit lines. While some of these deficiencies can becountered by the circuit designer, who expects that etching will not beentirely accurate, nevertheless, one can appreciate that inaccurateetching will limit the spacing of the circuit lines. When they are tooclose together, short circuits can occur or the current passing throughone circuit line can influence adjacent circuit lines. Consequently,accurate etching is important if increased circuit densities are to beachieved. However, the results of etching are largely influenced by thestructure of the copper foils.

The invention is particularly useful in the tape automated bondingprocess (TAB), but it has application in conventional printed circuitboards. The need for higher circuit densities has been growing in TABapplications and next-generation TAB tapes should accommodate 320channel, 50 μm pitch (spacing) circuits. Thus, more precise etchingbecomes ever more essential, to provide adequate insulation between thecircuit lines and to produce sharply defined profiles.

Another problem to which TAB tapes are subject is that in multi-layertapes the high temperatures used ( about 1 80° C.) causes a drop in thetensile strength of the copper. This can result in bending of the innerleads. Thus, higher tensile strength copper foils are desirable inaddition to the improved etching character discussed above.

Three related patents can be mentioned as having some relevance to thepresent invention. They are U.S. Pat. No. 5,403,465; 5,421,985; and5,431.803. In each patent, conditions are changed in the electrolyticprocess in order to change the tensile strength the elongation, and theroughness of the copper surface. In the '803 patent it is disclosed thatunder certain conditions uniform randomly oriented grains can be madeand that columnar crystals can be avoided. While such foils provedimproved tensile strength they have not been found to provide improvedetchability for making fine lines and spaces.

The present inventors had as an objective improving the etching factorof copper foil, to reduce the surface roughness on the matte side of thefoil, and to increase tensile strength. They have accomplished theirobjectives, as will be seen in the discussion below.

SUMMARY OF THE INVENTION

In one aspect, the invention is an electrodeposited copper foil whichhas a matte side roughness Rz of 2.5 μm or below and having an ambienttensile strength of 40 kgf/mm² or greater (measured after heating at180° C.). Such electrodeposited copper foils have copper crystals whichare found by x-ray diffraction analysis to have atomic cells withpredominently a (111) structure (a Miller index).

A copper foil of the invention will contain 50 to 1,200 ppm wt of tinand 1 to 50 ppm wt of iron.

The invention also is a process for making the improved electrodepositedcopper foils just described which is characterized by using anelectrolytic bath of copper sulfate and sulfuric acid containing 0.01 to0.10 g/l of a polyether glycol, 0.5 to 1.0 g/l of tin ions, 0.5 to 5.0g/l of iron ions, and less than 0.1 mg/l (wvt ppm) of choride ions. In apreferred embodiment, the polyether glycol is polyethylene glycol (PEG),the tin ion is derived from stannous sulfate, and the iron ion isderived from ferrous sulfate heptahydrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) and are drawings 1(b) illustrating exhibiting differentetching factors etched circuit lines.

FIG. 2 is an SEM photograph of the surface of the foil of the inventionfrom Example 1.

FIG. 3 is an SEM photograph of the surface of the foil from ComparativeExample 1.

FIG. 4 is an SEM photograph of the 50 μm pitch circuit lines of Example1.

FIG. 5 is an SEM photograph of the 50 μm pitch circuit lines ofComparative Example 1.

FIG. 6 is a diagram of the model used to calculate etching factor.

FIG. 7 is a diagram of the model used to calculate the linearity of theetched circuit lines.

FIGS. 8(a) and 8(b) show two x-ray diffraction charts, 8(a) for Example1 and 8(b) for Comparative Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Etching of Copper Foils

As previously explained, after copper foils are laminated to aninsulating substrate circuit patterns are made by photoimaging thepattern on the foil and the etching away of the unwanted portions of thecopper to leave circuit lines. The circuit designer would prefer thatthese circuit lines have a rectangular cross-sectional shape. However,the chemical etchants are not fully confined within the pattern definedby the photo-resist. They attack copper and remove it. Consequently,depending on the type of etchant and the time it is in contact with thecopper the shape of the circuit line will be affected.

FIG. 1 illustrates sections of circuit lines. In (a) the bottom of thecircuit line is reasonably linear. Since the bottom of the circuit linenext to the substrate is exposed to the etchant for a shorter time thanthe top, there is a tendency to form the trapezoidal shape shown. FIG.1(b) illustrates a poorly etched circuit line in which the top hasbecome narrower because the etchant removed copper under the edges ofthe protective layer representing the circuit pattern. The bottom edgeof the line is poorly defined. The top of the circuit line is narrowerthan that of FIG. 1a and consequently, the etching factor is poorer thanthe circuit line of FIG. 1a.

The term “etching factor” will be defined below. It provides a method ofcomparing the accuracy of etching. The method used is also shown in FIG.6. The definition of the edges of the circuit lines is determined bymeasurements according to FIG. 7. as will be discussed below in theExamples.

Making Electrodeposited Copper Foil

Foils are made on a commercial scale by electrodepositing copper fromsolution onto a rotating metal drum. Copper ions are supplied bydissolving copper metal in sulfuric acid. The electrolytic solution iscirculated between the drum (the cathode) and a closely-spaced anode. Anappropriate voltage is applied and the copper is deposited on the drumwith a suitable current density. Other materials are usually present inthe electrolytic solution, some intentionally added and others whichenter as impurities from various sources. including the copper metalwhich is dissolved to make up the solution. Since copper is continuallyremoved from the solution as it is plated out on the drum, the solutionis circulated and may be purified to remove impurities so they do notaccumulate. Makeup copper is added and the concentration of otherdesired ingredients are adjusted as necessary so that the copper foil isconsistent in quality. It is important in the present invention that thecomposition of the electrolytic bath be controlled carefully so thatcopper foil having the desired crystal structure and roughness isobtained. Copper foils have a smooth (shiny) side formed next to thedrum surface and a matte side (rough) which was in contact withelectrolytic solution. In most cases, the matte side is laminated to asubstrate since the rougher side of the foil adheres better than thesmooth side. It is one characteristic of foils of the invention that thematte side roughness is unusually low, 2.5 μm Rz or less. Ordinary foilshave much higher Rz values.

The electrolytic bath used to make copper foils of the inventionpreferably contains the following principal constituents:

CuSO₄ • 5 H₂O 283-334 g/l H₂SO₄ 110-200 g/l Polyether 0.01-0.10 g/l Tinion 0.5-1.0 g/l Iron ion 0.5-5.0 g/l Choride ion <0.1 mg/l

As will be seen in the examples below, the inventors have found that theamounts of the minor constituents are important if the desired foilproperties are to be obtained. The quality of the surface roughness andthe tensile strength are strongly influenced by the chloride ioncontent. The tensile strength at ambient temperatures is increased atlow chloride concentrations and the surface roughness is decreased. Themaximum should be 0.1 mg/l (0.1 wt. ppm). In commercial practice this isdifficult to do because chloride ions are present in water and in themetallic copper used to prepare the solutions.

A polyether glycol is included in a low concentration. It affects thetensile strength. At least 0.01 g/l is needed to maintain the improvedtensile strength while more than 0.10 g/l will increase the surfaceroughness of the foil. Polyethers glycols may be defined by the genericformula

HOR-O_(n)H

where:

R is lower alkyl

n is an integer up to about 200

A preferred polyether glycol is polyethylene glycol (mw 400-6000)

Copper foils have their tensile strength measured at ambienttemperatures. as made and after heating at 180° C. which may cause achange in the crystal structure and typically reduces the tensilestrength. Since the copper foil will ordinarily be exposed to hightemperatures during manufacture of printed circuits, this later tensilestrength is important, particularly when making multilayer TAB tapeswhere internal leads can be bent. The inventors have found that theaddition of small amounts of tin ions to the electrolytic solution willprovide a significant increase in the tensile strength after the foilhas been heated to 180° C. There should be at least 0.5 g/l in order toobtain a significant increase in tensile strength, but if the amountexceeds about 1.0 g/l then the surface roughness is increased beyond 2.5μm Rz.

It has been found that the concentration of iron ions in theelectrolytic solution affects the surface roughness. Iron should bepresent in an amount of at least 0.5 g/l. Above about 5 g/l the currentefficiency is reduced, although the surface roughness could be reducedfurther.

When an electrolytic solution according to the invention is used, theelectrical current density should be in the range of about 30 to 100A/dm², preferably about 40-65 A/dm², in order to produce a copper foilhaving a surface roughness Rz no greater than 2.5 μm. The operatingtemperature should be in the range of 35 to 60° C., preferably 45-55° C.Under these conditions, the copper foil not only has a relatively smoothmatte side, but has a high tensile strength after heating at 180° C. Thecopper crystals have atomic cells with predominantly a (111)orientation. This is believed to be the result of distributing tin oxideparticles of about 20 to 50° A in the copper. The resulting copper isvery fine grained. Such copper foils provide significantly improvedetching characteristics and retain a high tensile strength when exposedto high temperatures during processing into printed circuits. They areespecially useful in TAB tape applications. As will be understood bythose familar with metals, various crystallographic structures withinmetals and alloys may be described by Miller indices (e.g. (111). whichprovide a means of indicating the orientation of atomic cells within acrystal structure. (For example, see McGraw-Hill Dictionary ofScientific and Technical Terms, 4th ed. McGraw-Hill, Inc. 1989).

Since tin and iron compounds are added to the electrolytic bath tocontrol surface roughness and tensile strength after heating to 180° C.,they will appear also in the finished foil. Typically, they will amountto about 50 to 1,200 wt. ppm of tin and about 1 to 50 wt. ppm iron. Theymay be added to the electrolytic bath as soluble iron and tin compounds,such as ferrous sulfate heptahydrate and stannous sulfate.

EXAMPLE 1

An 18 μm thick copper foil was made by electrodepositing copper using acurrent density of 40-65 A/dm² and a temperature of about 45-55° C. Theelectrolytic solution contained 314 g/l of copper as CuSO₄ •5 H₂O, 150g/l of sulfuric acid 0.1 g/l of polyethylene glycol (6000 mw), 0.5 g/lof tin ion from stannous sulfate, and 1 g/l of iron from ferrous sulfateheptahydrate. The foil produced had a surface roughness Rz of 2.5, μm onthe matte side (measured by a commercial profilometer) and an ambienttensile strength of 41.1 kgf/mm² after heating at 180° C. for one hour.The matte side of this foil is shown in FIG. 2, a photomicrograph takenwith a scanning electron microscope (SEM). The very fine surfaceroughness can be clearly seen. FIG. 8(a) shows the results of an x-raydiffraction examination of the foil produced in this example. whichshows that it contains copper crystals having a major fraction of atomiccell planes with a (111) configuration. The remaining configurations,(220), (200) and (311) are found in smaller amounts.

A TAB tape was made by bonding the matte side of the foil to a polyimidesubstrate after giving the copper foil a conventional treatment toimprove adhesion. A fine circuit pattern having a pitch of 50 μm wasmade from the copper foil bonded to the polyimide film by conventionaletching. Examination of the circuit lines showed them to be very sharplydefined with only a small variation in width from the top to the bottom.Further, the definition of the bottom of the circuit lines wassatisfactory.

A further assessment of the quality of the circuit lines was made bymeasurements made using a photograph taken with 1000× magnificationusing an S-4100 scanning electron microscope (Hitachi Ltd.). See FIG. 4.The method used is illustrated in FIG. 7. A line was drawn parallel tothe edge of the circuit line at the top and bottom. The width of the topand bottom of the line (a and b) was measured at 10 points along theline at intervals of 10 μm. The standard deviation of the widthmeasurements was used to define the straightness of the top and bottomof the circuit. The results of this evaluation are shown in Table 1below. where they are compared with circuit lines made using a foilaccording to Comparative Example 1.

The etching factor was also determined for these circuit lines. Thisrepresents a calculation of a numerical value which indicates the degreeto which the circuit line approaches the ideal cross-sectional shape.The etching factor is determined by making the measurements shown inFIG. 6 and calculating the etching factor as Ef=2h b-a. Where the base band the top a are the same, then the etching factor becomes infinite. Inpractice, the base b is usually wider than the top a and Ef is a smallpositive number. Typically the base is wider than the top of the circuitline since the top is exposed to the etching agent for a longer period,thus creating a trapezoidal cross-section rather than the idealrectangular one. As with the measurement of the width described above,10 points were measured and the standard deviation of the valuesdetermined. The results are found in Table 1 below. It will be seen thatthe etching factor of the foil of the invention was larger and thestandard deviation smaller than those of the conventional foil ofComparative Example 1. The bending of inner leads on the TAB tape wasdetermined using an optical microscope. No bending was found with thisExample 1, but some bending was found with the foil of ComparativeExample 1.

Comparative Example 1

A TAB tape was made in the same manner as Example I except that, insteadof a foil of the invention, another 18 μm thick foil was used which wasdeveloped by Mitsui Mining & Smelting for use in making circuits with afine pitch. This foil, designated VLP, had a matte side roughness of 3.8μm Rz. Its tensile strength after heating for one hour at 180° C. was 48kgf/mm². The matte surface of the VLP foil can be seen in FIG. 3 andcompared with that of the foil of the invention shown in FIG. 2. Thedifference in roughness is clear. The X-ray diffraction analysis isshown in FIG. 8(b) where it can be seen that the predominent crystalatomic cell plane orientation is (220) rather than (111), in the foil ofthe invention, although other orientations are present.

A TAB tape was made using the same procedures as in Example 1 and thequality of the circuit lines evaluated in the same manner as Example 1.It was found that the circuit lines were less well defined as can beseen in FIG. 5. The results of the evaluation are shown in Table 1 wherethey can be compared with the results obtained with the foil of theinvention. The VLP foil made circuit lines which were less precise,having a poorer etching factor and greater variation from a straightline than were found with circuit lines made with the foil of Example 1.

TABLE 1 Ex. 1 Comp. Ex. 1 Average Top Width, μm 13.4 6.5 Average BottomWidth, μm 22.3 24.4 Etching Factor, Ef 4 2 Std. Deviation of Bottom fromstraight line 0.12 1.16 Std. Deviation of Top from straight line 0.120.20 Surface Roughness Rz μm 2.5 3.8 Bending of inner lead None partialPost-Heating Tensile Strength, kgf 1 mm² 41.1 48.0

EXAMPLES 2-5 AND COMPARATIVE EXAMPLES 2-7

A series of copper foils were made using a current density of about50-65 A/dm² and a bath temperature of about 45-55° C. The composition ofthe electrolytic bath was varied to examine the effects of changing theamounts of the polyether and the tin and iron ions. Except forComparative 7 each electrolytic bath contained less than 0.1 ppm wt ofchloride ion.The copper foils produced were examined for their surfaceroughness and their tensile strength, both at ambient temperature, asmade and after being heated for one hour at 180° C. The results of thesetests are shown in Table 2, which also includes the results for the foilof Example 1 and Comparative Example 1.

TABLE 2 Examples Comparative Examples 1 2 3 4 5 1 2 3 4 5 6 7Electrolytic Bath CuSO₄.5 H₂O, g/l 314 314 314 314 314 — 314 314 314 314314 314 H₂SO₄, g/l 150 150 150 150 150 — 150 150 150 150 150 150Polyether, g/l 0.1 0.01 0.05 0.10 0.10 — — 0.01 0.05 0.10 — — Tin, g/l0.5 0.5 0.5 1.0 1.0 — — — — — — — Iron, g/l 1.0 0.5 1.0 1.0 5.0 — 1.01.0 1.0 1.0 — — Cl, ppm <0.1 <0.1 <0.1 <0.1 <0.1 — <0.1 <0.1 <0.1 <0.1<0.1 1 Foil Properties Tensile Strength, kgf/mm + 2 ambient 52.0 55.754.1 54.0 53.8 52.0 64.6 71.2 70.2 70.1 53.0 46.3 after 180° C. 41.140.5 40.8 48.8 48.5 48.0 30.1 30.1 29.5 29.6 28.0 — Elongation, %ambient 4.5 4.8 4.5 4.5 4.7 7.0 3.8 3.7 4.2 4.2 4.9 13.2 after 180° C.6.6 6.2 7.6 5.6 5.9 6.0 13.8 15.5 15.0 16.2 15.0 — Matte Side 2.5 2.42.5 2.4 2.5 3.8 2.1 2.0 2.1 2.0 4.5 9.6 Roughness, Rz μm

Several conclusions can be drawn from the data presented in Table 2.Addition of tin to the electrolysis bath appears to decrease the tensilestrength of the copper foil at ambient temperature, but to reduce theloss of tensile strength after heating to 180° C. the elongation of thecopper foil is lower than conventional foil. Iron appears to reducesurface roughness (see Comparative 6). Chloride ion also affects surfaceroughness (see Comparative 7).

It is believed that the copper foils of the invention are superiorbecause the changes to the composition of the electrolysis bath causethe planes in the atomic cells of the copper crystals to be orienteddifferently than in more conventional foils. As shown in FIG. 8 the newfoils have their crystals with (111) orientation predominent. This mayaccount for the reduced surface roughness and may help to explain whythe new foils can be etched more accurately.

What is claimed is:
 1. An electrodeposited copper foil containing50-1,200 wt.ppm of tin and 1 to 50 wt. ppm of iron, said foil having amatte side roughness R_(z) of 2.5 μm or less, a tensile strength of atleast 40 Kgf/mm² after heating for one hour at 180° C., and aselectrodeposited, copper crystals having atomic cells with a (111)orientation as measured by x-ray diffraction.
 2. An electrodepositedcopper foil of claim 1 produced by electrolysis of a copper solutioncontaining 0.01 to 0.10 g/l of a polyether glycol, 0.5 to 1.0 g/l of tinion, 0.5 to 5.0 g/l of iron ion, and less than 0.1 mg/l chloride ion. 3.An electrodeposited copper foil of claim 2 wherein said copper solutioncontained polyethylene glycol, stannous sulfate, and ferrous sulfateheptahydrate.
 4. An electrodeposited copper foil of claims 2 whereinsaid electrolysis was carried out at a current density of about 30-100A/dm² and at a temperature of about 35 to 60° C.
 5. A TAB tapecomprising circuit lines made by laminating the copper foil of claim 1to a polyimide substrate.
 6. A printed circuit board comprising circuitlines made by laminating the copper foil of claim 1 to a substrate andetching said copper foil.
 7. An electrodeposited foil containing50-1,200 wt. ppm of tin and 1-50 wt. ppm of iron, said foil having amatte side roughness R_(z) of 2.5 μm or less, a tensile strength of atleast kgf/mm² after heating for one hour at 180° C., and copper crystalshaving atomic cells with a (111) orientation as measured by X-raydiffraction, said electrodeposited foil being obtained byelectrodepositing at a current density of about 30-100 A/dm² from acopper solution containing copper ions, 0.01 g/l of a polyetherglycol,0.5-1.0 g/l of tin ions, 0.5-5.0 g/l of iron ions, and less than 0.1mg/l chloride ion.
 8. An electrodeposited foil of claim 7 wherein saidcopper solution contains polyethylene glycol, stannous sulfate, andferrous sulfate heptahydrate.